1. Field of the Invention
This invention relates, in general, to a process for bonding the surfaces of two materials and more particularly to a process for bonding the surfaces of the light coupling material and the encapsulating material of an optocoupler to preclude gaps from forming therebetween, thereby inhibiting air and moisture, and therefore electric current, from passing therebetween.
2. Background Art
The number of industrial applications calling for the bonding of the surfaces of two polymeric materials is pervasive One such application is optocouplers used for electrical isolation in systems such as computers, power supplies, telecommunications, and controllers. Optocouplers typically comprise a Light Emitting Diode (LED) electrically connected to one or more electrodes and a light sensing diode electrically connected to one or more additional electrodes. The LED, light sensing diode and portions of the electrodes are enclosed in a light coupling material The light coupling material is then enclosed in an encapsulating material. The light coupling material typically comprises a clear silicone gel that allows light to pass freely from the LED to the light sensing diode, while maintaining electrical isolation between the electrodes. The encapsulating material typically comprises an opaque epoxy that reflects light from the LED back into the clear gel and prevents light from external sources from reaching the clear gel. The encapsulating material also serves as a protective enclosure from external mechanical forces.
However, since the light coupling material and encapsulating material are different in composition and as many as 15,000 volts or more may appear across the electrodes, an electrical breakdown occurs between the electrodes along the boundary surface of the light coupling material and the encapsulating material. The light coupling material and encapsulating material are heated during the manufacturing process and the light coupling material will typically have a coefficient of expansion many times greater than the encapsulating material. During cooling after encapsulation, the light coupling material will therefore contract more than the encapsulating material, causing gaps to form at the interface between the two materials. This gap comprises gas or air and has a lower dielectric strength than the light coupling material and the encapsulating material. Over time, moisture condenses along the electrodes to the boundary between the two materials and into the gaps, thereby substantially increasing the likelihood of electrical breakdown.
One known method of reducing the effect of this electrical leakage path, or separation, is to provide a layer of dielectric strength enhancing material such as glass frit at the boundary between the two materials. The glass particles are placed irregularly and so that each glass particle extends into both materials. However, this method does not improve bonding and does not impede electrical breakdown when a gap forms at the boundary of the two materials. It increases the electrical leakage path length and results in a higher isolation voltage. Air and moisture will fill the gap, providing a path of least resistance for the current. Furthermore, the manufacturing process is substantially complicated by this procedure.
Another known method comprises roughening the surface of the light coupling material thereby increasing the arc length path of current flowing therealong. However, this method increases manufacturing processes and does not increase adhesion of the two materials at their interface.
Yet another known method of reducing electrical breakdown comprises providing an electrically insulating, transparent film on one or more of the electrodes, the LED, and the light sensing diode. However, this procedure does not increase bonding. Manufacturing complexity is increased and air and moisture still can seep between the film and the encapsulating material to the boundary between the light coupling material and the encapsulating material.
Thus, what is needed is a process for adhesively bonding the surfaces of two materials for preventing the materials from separating from each other, preventing air and moisture from seeping along the boundary, and preventing electric current from passing along the boundary interface from one electrode to another electrode.